The present invention relates to electrical connectors. More particularly, the present invention relates to ball grid array sockets.
Ball Grid Array (BGA) socket devices are generally known for electrically connecting an electrical array device, such as an IC device, to an external circuit, such as a printed circuit board (PCB). Traditionally, these sockets have connecting pins configured so as to correspond to the connecting terminals of various types of electrical devices. For example, electronic devices or IC packages having leads adapted for use in a ball grid array (BGA) having nearly spherical connecting terminals or for use in a land grid array (LGA) having pad-like connecting terminals are known. These packages have a relatively low height which is desirable because they save space in electronic assemblies.
The BGA type IC package typically consists of a package body incorporating required circuits, and a large number of connecting terminals that are made of a solder material and arranged on a bottom surface of the package body in a grid-like array pattern spherically protruding for connection to an external circuit. For electrically connecting the BGA type IC package to an external circuit, such as a PCB, the package body is typically mounted at a predetermined location on the PCB and the connecting terminals are connected directly to corresponding circuit terminals by partially melting the solder material of the connecting terminals. A drawback, however, of using solder bonding is the package is not easily removable for replacement or upgrade. Consequently, it is desirable to provide an electrical connector for mounting an electronic device on the circuit board in a removable or separable fashion.
Electrical connectors for removably mounting a BGA or LGA package to a PCB are generally known. These removable connectors typically comprise a substantially flat dielectric housing that resides between the electronic device and the PCB. The housing has an array of cavities in which electrical contacts are disposed. The contacts are arranged in correspondence with the array of leads of the electronic device. Each of the contacts typically has a pair of oppositely extending tail portions that project beyond external surfaces of the connector housing. When the electronic device is mounted on the connector, each of the contacts has one tail portion engaged with a respective lead of the electronic device and the opposite tail portion engaged with a respective lead or pad on the PCB.
Typically, the electronic device is removably held on the socket housing by a compressive force which presses down on the electrical contacts of the socket from the top, thereby causing lateral deflection of the contact as the electrical connection is formed between the electronic device and the PCB. Alternatively, traditional sockets have a camming mechanism for selectively moving the electrical contacts of the socket in a lateral direction into or out of engagement with the sides of the leads of the electronic device. This lateral deflection requires space in the socket housing and can dictate minimum spacing between electrical contacts. This problem is compounded as electronic devices become smaller and smaller and as array densities associated with the electronic devices continue to increase
Also, conventional socket assemblies do not always provide a reliable mechanical/electrical connection between the electronic device and the socket assembly. One problem is that manufacturing variations in the production of certain components allow for lateral deflection between the components. Such deflection adds stress on the components that can lead to device damage and a loss of proper electrical connection.
Another problem associated with the use of socket assemblies is the difference in how each component reacts to temperature changes. The coefficient of thermal expansion (CTE), as used herein, represents the magnitude of dimensional change of a material over a specific temperature range. By way of example, among an electronic device, a socket, and a PCB, a different CTE will exist as between the connector material, the material of the solder joint, the socket material, and the PCB material. Area array components (connectors/devices/packages, etc.) are typically limited in size based on the effects of the differential CTE among the associated materials. As such, the greater the differential displacements according to the CTE mismatch during thermal changes, the greater concern for the electrical integrity and performance of the system.
Consequently, there is a need for a ball grid array socket that provides for an electrical connection between the socket assembly and the electronic device while also maintaining a reliable mechanical connection in the event of component movement.
The present invention satisfies the aforementioned need by providing systems and methods that provide a solid and reliable connection between a socket assembly and an electronic device secured thereto. In accordance with the present invention, a socket assembly is provided for connecting an electrical device to an electrical circuit, such as, for example, a printed circuit board. The socket assembly comprises a housing having a bottom surface and top surface with a plurality of apertures therethrough, a plurality of contact terminals. Each terminal is disposed within one of the apertures of the socket, and a compressible ball of electrically conductive material disposed within each of the plurality of terminals. As such, when the device is placed in the socket, the contacts of the device depress the compressible ball in the contact terminals and provide an electrical connection and create mechanical play between the terminals and the device contacts. Consequently, if movement occurs between the electrical device and the socket (from thermal expansion or the like), the compressible ball of electrically conductive material will absorb the movement, thereby absorbing any stresses and strains caused by such movement. Additionally, in one embodiment of the present invention, the socket assembly includes a loading device for securing the electrical device to the socket.